Various types of optical spectrometers are in use for such purposes as atomic emission spectroscopy, atomic absorption spectroscopy and astronomy. A complete system generally consists of a source of radiation, a spectrometer for separating and detecting individual spectral components, and a data station for processing the information from the spectrometer. The radiation source, for example, may be a system for injecting a test sample into an inductively coupled plasma where the atomic species in the sample are excited to radiate characteristic atomic emission. As another example, a sample is evaporated in a graphite furnace where the gaseous sample absorbs certain frequencies of the incident radiation to provide atomic absorption lines. Similarly, astronomical sources provide atomic emission and absorption lines.
The type of spectrometer of particular interest herein involves sequential measurement utilizing a monochromator in which a grating or prism is rotated to direct a narrow portion of the spectrum to a slit and a detector. The angle is adjusted to correspond to the different emission (or absorption) lines of the elements. A single detector is used, either a solid state detector or a photomultiplier tube. The measurement process involves rotation of the grating with measurements at each of a series of selected locations corresponding to grating angles appropriate to the atomic emission lines.
Sophisticated monochromators, particularly of the type used for quantitative analysis of atomic elements in samples injected through an induction coupled plasma, are controlled by microprocessors and personal computers. Such a system is typified by a Model Plasma 40 emission spectrometer sold by The Perkin-Elmer Corporation, Norwalk, Conn., and described in co-pending U.S. patent application Ser. No. 837,438 filed Mar. 7, 1986, issued as U.S. Pat. No. 4,779,216 (Collins) assigned to the assignee of the present application. A stepper motor orients a grating with respect to the slit of the detector to locate any selected portion of the spectrum for measurement of the intensity of that portion. A dedicated microprocessor provides a suitable signal to the motor for selective orientation in relation to wavelength. The microprocessor also receives the intensity signal from the detector, and provides data in the form of spectral intensity vs spectral position. In practice a scanning signal is provided to the motor to sequentially scan the spectrum in a series of steps.
In order to allow a reasonably fast scan, signals to the motor are such as to scan in spectral windows which are just wide enough to encompass each of the selected spectral bands with some margin. The motor scans through all steps in a window, and then moves quickly to the next window before scanning in steps again, and on to the next window, etc., for the whole series of spectral bands.
The aforementioned Collins reference discloses a system for initially calibrating the window positions for the desired spectral lines in a monochromator to compensate for mechanical imperfections in its diffraction grating and grating drive assembly. The disclosed system employs a two-stage interactive procedure. Each stage involves measuring position errors of lines for a standard element and fitting these errors to a quadratic polynominal by the least squares method, as a function of window position. An iterative, self-consistent, discrete Fourier transform is used for the determination of multiple positioning correction terms. When the Fourier calculations are completed, the results of the calibration procedure are presented by the system to the analyst for acceptance. If accepted, the positioning error of the primary calibration line is measured, stored and used by the system to establish a zero centered distribution of positioning errors each time the monochromator is reinitialized.
However, the above-described calibration may be insufficient, especially for long-term operations. Temperature changes cause minute distortions in the spectrometer resulting in drifting of the positions of the spectral bands with respect to their windows. Without compensation, over a period of time a peak may drift through an edge of its window resulting in erroneous, undetected data. As temperature controlled rooms are often impractical or insufficient, a common method for minimizing drift is to temperature control the monochromator with a built-in heater and optionally a thermostat. Such a temperature control system has been found to be less than satisfactory because it is difficult to control all components uniformly without adding substantial cost and complexity to the apparatus.